This invention relates to the pointer shaft in an indicating instrument, and more particularly to the pointer shaft in an indicating instrument which comprises: a movement for turning the pointer shaft according to an amount of measurement; a pointer having an engaging hole in the base end portion, into which the end portion of the pointer shaft is fixedly pushed; a dial which is, for instance, graduated, for indicating the amount of measurement in cooperation with the pointer of the pointer shaft, and which is installed, for instance, on a motor vehicle and is suitable as an indicating instrument such as a speed meter, a tachometer or boost meter to be installed on a vehicle.
One example of a conventional indicating instrument of this type is a cross coil type indicating instrument which is designed as shown in FIG. 3. In the indicating instrument, current changing according to an amount of measurement is applied to a pair of coils to allow the latter to form magnetic fields perpendicular to each other, and a magnet rotor is rotated in the direction of the composite magnetic field which is formed by combining the magnetic fields of the two coils, to turn the pointer on the dial to indicate an amount of measurement. This will be described in more detail.
As shown in FIG. 3, a movement 1 for turning a pointer shaft 1a according to an amount of measurement has a coil bobbin 11, on which two coils 12a and 12b are wound in such a manner that they are perpendicular to each other. A disk-shaped magnetic rotor 13 with S and N poles is arranged in the space defined by the coil bobbin 11. The magnetic rotor 13 is fixedly mounted on a rotary shaft 14 of made metal. More specifically, the rotary shaft 14 penetrates the magnetic rotor 13 at the center in such a manner that it is perpendicular to the magnet rotor. The rotary shaft 14 is rotatably supported by the coil bobbin 11 in such a manner that one end portion of the rotary shaft 14 extends outside the coil bobbin 11, thus serving as the aforementioned pointer shaft 1a. When current is applied to the coils 12, the magnetic rotor 13 together with the pointer shaft 1a is turned through a predetermined angle in response to the applied current.
A spring ring 15 is fixedly mounted on the pointer shaft 1a. The inner end portion of a spiral hair spring 16 is secured to the outer cylindrical wall of the spring ring 15, and the outer end portion is secured to a frame 22. The hair spring 16 function as follows: That is, when the coils 12 are not energized and therefore the magnetic rotor is not turned, the pointer shaft la is returned to the "0" position and held there by the elastic force of the hair spring 16.
A dial 18 is mounted on the movement 1 with screws 19. The upper end portion of the pointer shaft 1a protrudes from the upper surface of the dial 8 such that a pointer 20 made of synthetic resin may be fixedly engaged. Specifically, the end portion of the, pointer shaft la is tapered so that it can be readily inserted into an engaging hole 20b formed in the base end portion 20a of the pointer 20.
A movement casing 21 is provided outside the coil bobbin 11 so as to isolate the inside and outside magnetic fields from each other.
In the above-described indicating instrument, the pointer 20 is fixedly mounted on the upper end portion of the pointer shaft 1a as follows: First, the pointer 20 is temporarily mounted on the upper end portion of the pointer shaft 1a. Under this condition, current corresponding to a predetermined amount of measurement is applied to the cross coils 12. For instance, in the case where the instrument is to be used as a speed meter, current corresponding to 40 km is applied to the cross coils 12. With the current supplied in this manner, the position of the pointer 20 temporarily mounted on the upper end portion of the pointer shaft is so adjusted that the pointer indicates the amount of measurement on the dial 18. Under this condition, the upper end portion of the pointer shaft 1a is pushed into the engaging hole 20b of the pointer 20. Then, the pointer 20 thus adjusted in position is fixedly mounted on the pointer shaft.
When a predetermined current is applied to the coils 12 according to predetermined measurement signal such as a vehicle speed, the coils 12 form magnetic fields. The magnetic fields thus formed are combined into a composite magnetic field. As a result, the magnetic rotor 13 and accordingly the pointer shaft la is turned according to the strength of the composite magnetic field; i.e., the amount of measurement. In other words, the pointer is turned to the position on the dial 18 which corresponds to the amount of measurement.
When the application of the current to the coils 12 is suspended, the magnetic rotor 13 is returned to the "0" position by the elastic force of the hair spring 16.
As was described above, in the conventional indicating instrument, the pointer 20 is secured to the end portion of the pointer shaft la as follows: First, current corresponding to a predetermined amount of measurement is applied to the cross coils 21. Under this condition, the position of the pointer is so adjusted that the pointer indicates the corresponding value on the dial 18, and then the pointer 20 is depressed so that the end portion of the pointer shaft 1a is pushed into the engaging hole 20b of the pointer. Hence, if the surface of the pointer shaft, or the inner wall of the engaging hole 20b has scratches, or the engaging hole 20b is distorted even slightly, then the pointer shaft 1a and the pointer 20 will turn relative to each other when the pointer is depressed so as to be fixedly mounted on the pointer shaft.
As a result, the position of the pointer 20 is no longer accurate. Therefore, the pointer 20 indicate the correct measurement value.